Composite structures are fabricated with a core sandwiched between skins which may be laminated layers. The core is generally porous, being made of a honeycomb or structural foam (thermoset, thermoplastic, or non-polymeric) for both favorable weight and strength characteristics. The core may contain various fillers and additives including foaming agents, carbon or glass hollow microspheres or fibers, coupling agents, flame retardants, sound suppression particles, conductive materials and antimicrobia.
The core is normally fabricated or molded to as near final dimensions as possible because the machining of core materials is an expensive, labor intensive process. Further, any dimensional deviation of the core may cause intrusion or extrusion of the bond line at the interface of the core and skins. Intrusion can produce voids and thick bond lines and extrusion may result in an inadequate sandwich structure.
The sandwich structure may include fabrics such as laminated fiber reinforced skins, mats, braids, and woven fabrics, all of which surround the core. It is conventional to use polymer pre-impregnated, commingled carbon or glass matrices so to assist in consolidation and strength of the sandwich structure. These commingled materials are relatively expensive.
Hybrid sandwich structures include at least one high impact layer such as a layer of a metallic or ceramic monolithic or composite skin. Presently there are several military and commercial applications which require hybrid laminated sandwich structures such as armored vehicles, military helmets, aircraft structures and other applications which need shielding from electrical disturbances or which require high thermal conductivity. It is known that a multi-layer metallic and/or ceramic composite sandwich structure having a foam core is most effective to stop penetration by a projectile such as a bullet. When a projectile contacts the structure, the energy is dissipated through the different layers to lessen both the damage and impact to the instrumentality being protected by the sandwich structure.
In the above applications the core and skins are generally made separately and then bonded together. Any other layers including the metallic or ceramic layers are then bonded to the sandwich, normally by an adhesive. Because of the use of adhesives having different thermal and physical properties from the other components or layers of the sandwich structure, the adhesive bond line becomes the weakest section of the component and detachment at the bond lines may cause separation of the sandwich structure when it is subject to thermal or load stresses. The separation of a layer or layers of the sandwich structure may cause severe degradation of the component's performance and in some cases can directly cause the loss of the use of the component completely.
In using an adhesive such as an epoxy, the type of polymeric material used in the sandwich structure is limited as most adhesives cannot withstand high temperatures used for processing high temperature thermoplastics such as polyetheretherketone (PEEK), which has a processing temperature of about 750.degree. F.
The fabrication cost of hybrid structures is prohibitive as the current processes for forming sandwich structures, such as injection molding, having relatively low production rates and are not as easily adaptable for custom design. Further, as described earlier, the current processes require secondary operations such as the machining and grinding of the core and the adhering of layers onto the sandwich structures.
Dealing with the fabrication process of sandwich structures is expensive. Indeed according to one estimate related in composite manufacture for use in helicopter fuselages, fabrication accounts for approximately 60% of fuselage formation costs with the remaining 40% being relatively equally divided between the cost of materials, quality assurance and assembly. Reduction of the costs associated with any of those items would be advantageous.
The present invention is directed to overcoming one or more of the above problems.